The present invention relates to electrodeposited copper foil with carrier predominantly employed for producing printed wiring boards.
Conventionally, electrodeposited copper foil with carrier has been employed as a material for producing printed wiring boards, which are widely used in the electric and electronic industries. In general, electrodeposited copper foil with carrier is bonded, through hot-pressing, onto an electrically insulating polymer material substrate such as glass-epoxy substrate, phenolic polymer substrate, or polyimide, to thereby form a copper-clad laminate, and the thus-prepared laminate is used for producing printed wiring boards of high density mounting.
In hot-pressing, a copper foil, a prepreg (substrate) which is cured into a B-stage, and mirror plates serving as spacers are laid-up in a multilayered manner, and the copper foil and the prepreg are hot-press-bonded at high temperature and pressure (hereinafter the step may be referred to as xe2x80x9cpress-formingxe2x80x9d). When wrinkles are in turn generated in the copper foil to be pressed, cracks are generated in the wrinkled portions, thereby possibly causing bleeding of resin from a prepreg, or open circuit of a formed electric circuit during an etching step followed in production steps of printed wiring boards. In an electrodeposited copper foil with carrier, the carrier foil prevents generation of wrinkles in the electrodeposited copper foil.
Electrodeposited copper foils with carrier are generally divided into two types; i.e., foils with peelable carriers and foils with etchable carriers. Briefly, the difference between the two types of foils lies in the method for removing the carrier after completion of press-forming. In foil with peelable carrier, the carrier is removed by peeling, whereas in foil with etchable carrier, the carrier is removed by etching. The present invention is directed to electrodeposited copper foil with peelable carrier.
However, the peel strength of conventional peelable carriers after completion of press-forming varies considerably, and a preferable strength of 50-300 gf/cm is generally required. In some cases, a carrier foil cannot be removed from the copper foil. Thus, conventional peelable carriers have a drawback; i.e., target peel strength is difficult to attain. The drawback prevents the widespread use of the electrodeposited copper foil with carrier employed for general use.
Causes of variation in peel strength of a carrier foil will next be described. Conventional electrodeposited copper foil with carrier, regardless of whether the carrier is peelable or etchable, has a metallicxe2x80x94e.g., zinc-containingxe2x80x94adhesive interface layer between the carrier foil and the electrodeposited copper foil. The amount of metal components forming the adhesive interface layer determines, with slight dependence on the type of the carrier foil, whether the formed copper foil with carrier has peelable carrier foil or etchable carrier foil.
In many cases, such a metallic adhesive interface layer is formed electrochemically; i.e., through electrodeposition by use of a solution containing a predetermined metallic element. However, in electrodeposition, controlling the amount of deposition on a very minute scale is difficult, and reproduction of the deposition is unsatisfactory as compared with other methods for forming the adhesive interface layer. In addition, the boundary line of the required deposition amount determining whether the formed carrier becomes peelable or etchable is difficult to adjust; i.e., small variations in amount of a metallic component contained in the adhesive interface layer determine the type of the carrier. Thus, stable peeling performance may be difficult to attain.
From another point of view, such a carrier foil is removed by peeling after completion of press-forming, typically at a temperature as high as 180xc2x0 C. under high pressure for 1 to 3 hours. Components contained in the carrier foil and copper atoms contained in the electrodeposited copper foil may be mutually diffused through the adhesive interface layer. Such mutual diffusion strengthens the adhesion, thereby failing to attain moderate peel strength.
In order to solve the aforementioned drawbacks, the present inventors have proposed electrodeposited copper foil with carrier in which the adhesive interface layer between the carrier foil layer and the electrodeposited copper foil comprises an organic agent such as CBTA, and a method for producing the electrodeposited copper foil with carrier.
The aforementioned electrodeposited copper foil with carrier which the present inventors have proposed completely solves the drawback that the carrier foil cannot be peeled; i.e., the proposed foil can be peeled at a strength of 3-200 gf/cm. However, there has been still increasing demand for a copper foil which can be peeled with a moderate and constant peel strength after a copper-clad laminate is produced by use of an electrodeposited copper foil with carrier.
Meanwhile, an advantage of electrodeposited copper foil with carrier per se is the state where one surface of the carrier foil are placed as if it were bonded in a lamination manner to one surface of an electrodeposited copper layer. In other words, the electrodeposited copper foil with carrier can prevent staining the surface of the electrodeposited copper foil with foreign matter and damaging the electrodeposited copper foil layer by maintaining the bonding state at least immediately before an etching step for forming printed circuits, which step is carried out after production of a copper-clad laminate through hot-pressing the electrodeposited copper foil with carrier and a prepreg (substrate).
Thus, separation of a carrier foil and an electrodeposited copper foil during handling of the electrodeposited copper foil with carrier before hot-press-forming is not acceptable. Although the carrier foil must be peeled with a moderate peel strength after completion of hot-pressing, lamination-type bonding of the carrier foil to one surface of an electrodeposited copper foil of a copper-clad laminate must also be maintained, at least immediately before an etching step so as to prevent contamination and staining the surface of the copper clad laminate with foreign matter.
In view of the foregoing, the present inventors have conducted extensive studies, and have concluded that the peel strength between a carrier foil and an electrodeposited copper foil should be controlled to 3 gf/cm to 100 gf/cm so as to maintain lamination-type bonding of the carrier to one surface of the electrodeposited copper foil at least immediately before an etching step with lower peel strength.
Thus, the aforementioned demands can be satisfied by selecting combination of materials of a carrier foil and an electrodeposited copper foil, which materials are predominant materials for forming an electrodeposited copper foil with carrier. This approach differs from the approach of modifying an organic agent which is employed in an adhesive interface layer and the approach of improving interface-forming techniques, such as a method for forming the adhesive interface layer. Since an electrodeposited copper foil with carrier is hot-pressed during production of a copper-clad laminate, the copper foil with carrier is subjected to a certain amount of thermal stress. The present inventors have found that, among properties of the materials, coefficient of thermal expansion is an important factor. The present invention has been accomplished on the basis of this finding.
Accordingly, the present invention provides an electrodeposited copper foil with carrier comprising a carrier foil layer, an organic adhesive interface layer formed on the carrier foil layer, and an electrodeposited copper foil layer formed on the organic adhesive interface layer, wherein the difference between the coefficient of thermal expansion of material forming the carrier foil layer at a certain temperature and that of material forming the electrodeposited copper foil at the same temperature is 4xc3x9710xe2x88x927/deg.C or more.
After careful studies, the present inventors have found that a carrier foil of the electrodeposited copper foil with peelable carrier employed for producing copper-clad laminates can be peeled considerably easily when the difference between the coefficient of thermal expansion of material forming the carrier foil layer at a certain temperature and that of material forming the electrodeposited copper foil at the same temperature is 4xc3x9710xe2x88x927/deg.C or more. Thus, the invention is based on this finding. When the carrier foil layer and the electrodeposited copper foil layer are subjected to heat hysteresis and the two layers exhibit identical thermal expansion behavior, the bonding conditions between the two layers via the organic adhesive interface layer are maintained within an elastic limit. Under such conditions, peeling at the organic adhesive interface layer is not promoted. However, when the difference between the coefficient of thermal expansion of material forming the carrier foil layer at a certain temperature and that of material forming the electrodeposited copper foil at the same temperature is 4xc3x9710xe2x88x927/deg.C or more, thermal stress for causing shear of the two layers at the organic adhesive interface is generated by heat hysteresis, which typically occurs during a process for producing copper-clad laminates. Thus, the two layers can be peeled from each other much more easily. When the difference between the coefficient of thermal expansion of material forming the carrier foil layer at a certain temperature and that of material forming the electrodeposited copper foil at the same temperature is controlled to 4xc3x9710xe2x88x927/deg.C or more, the peel strength can be controlled to 3-100 gf/cm, which is a target peel strength in the present invention. The difference, i.e., 4xc3x9710xe2x88x927/deg.C or more, may be applied in either case of expansion or shrinkage of the carrier foil with respect to the electrodeposited copper foil.
In the present invention, the range xe2x80x9c4xc3x9710xe2x88x927/deg.C or morexe2x80x9d does not refer to a range in which the upper limit remains uncertain. This is because, given a material forming the carrier foil layer and a temperature at which the foil is treated, a specific upper limit of the difference between a coefficient of thermal expansion of material forming the carrier foil layer and that of material forming the electrodeposited copper foil is univocally determined.
In the electrodeposited copper foil with carrier of the invention, an organic adhesive interface layer is formed on a carrier foil layer and an electrodeposited copper foil layer is formed on the organic adhesive layer. Accordingly, the organic agent adheres to both the carrier foil layer and the electrodeposited copper foil layer, and the layer containing the organic layer also serves as an adhesive interface layer. When an appropriate organic agent is employed in the adhesive interface disposed between the carrier foil layer and the electrodeposited copper foil layer, peeling behavior of the carrier foil layer and the electrodeposited copper layer caused by difference in coefficient of thermal expansion is relaxed, even though the electrodeposited copper foil with carrier is subjected to certain thermal impact during a process for producing copper-clad laminates. Accordingly, spontaneous peeling of the carrier foil layer and the electrodeposited copper layer is considered to be prevented.
In the present invention, the electrodeposited copper foil with carrier has a schematic cross-sectional structure as shown in FIG. 1. Specifically, one surface of the carrier foil layer (hereinafter may be simply referred to as xe2x80x9ccarrier foilxe2x80x9d) is placed as if it were boned in a laminated manner to one surface of the electrodeposited copper layer (hereinafter may be simply referred to as xe2x80x9celectrodeposited copper foilxe2x80x9d) via the organic adhesive interface layer. Typically, such as electrodeposited copper foil with carrier and a prepreg (e.g., FR-4 substrate) or an internal printed wiring boardxe2x80x94the prepreg and the internal printed wiring board serving as insulating layersxe2x80x94are laminated, and the resultant laminate is press-formed in an atmosphere at approximately 180xc2x0 C., to thereby obtain a copper-clad laminate.
In the present invention, either organic material or inorganic metallic material may be used to form the carrier foil which is combined with an electrodeposited copper foil, so long as the difference in coefficient of thermal expansion is 4xc3x9710xe2x88x927/deg.C or more. However, as described in the invention, an electrodeposited copper foil is advantageously employed, in view of ease of recycling the foil and stable production thereof. In this case, although the electrodeposited copper foil and the carrier foil of the electrodeposited copper foil with carrier of the present invention are both electrodeposited copper foils, copper foils having different physical properties, particularly coefficient of thermal expansion, must be combined.
In order to provide better understanding of the following description, types of electrodeposited copper foils will next be described. Although there are a variety of international standards regarding the classification of electrodeposited copper foils, classification on the basis of the most widely employed standards; i.e., IPC (The Institute for Interconnecting and Packaging Electronic Circuits) standards, will be described.
According to the IPC standards, electrodeposited copper foils are classified as Grade 1 to Grade 3 on the basis of basic physical properties such as elongation and tensile strength. Copper foil designated by Grade 1 is standard electrodeposited copper foil, and copper foil designated by Grade 2 is high ductility electrodeposited copper foil. These days, among persons having ordinary skill in the art, electrodeposited copper foils belonging to Grades 1 and 2 are generally called standard electrodeposited copper foils (hereinafter these copper foils are referred to as xe2x80x9cstandard electrodeposited copper foilsxe2x80x9d). Electrodeposited copper foil belonging to Grade 3 is generally called HTE foil. HTE foil generally refers to copper foil exhibiting high temperature elongation of 3% in an atmosphere at 180xc2x0 C. HTE foil is completely different from standard copper foils belonging to Grades 1 and 2, since the standard copper foils exhibit a high temperature elongation less than 2%.
In recent manufacture of printed wiring board, copper foils belonging to Grade 3 are further classified clearly into two categories; i.e., electrodeposited copper foils exhibiting a high temperature elongation of approximately 3% to 18% (hereinafter simply referred to as HTE foils) and electrodeposited copper foils exhibiting a high temperature elongation of approximately 18% to 50% (throughout the present description, these foils are simply referred to as S-HTE foils). These two types of foils are employed in accordance with purposes of use.
The basic difference between HTE foil and S-HTE foil lies in characteristics of deposited crystals, even though these two foils comprise electrodeposited copper having a purity of approximately 99.99%. During a process for producing copper-clad laminates, an electrodeposited copper foil is hot-pressed so as to be laminated with a substrate by heating at 180xc2x0 C. for approximately 60 minutes. Through observation under an optical microscope of the metallographic structure of the foils after completion of heating, no recrystallization is observed in HTE foil, but recrystallization is observed in S-HTE foil.
The difference is considered to be due to production conditions of the foils. Briefly, production conditions during electrolysis, such as composition of a solution, concentration of a solution, a method for filtering a solution, solution temperature, additives, and current density, are modified in order to control physical properties of copper foils. This may cause variation in crystallographic properties of deposited crystals. Particularly, the more easily recrystallization occurs, the more dislocations are accumulated in crystals. The dislocations are immobilized tightly, and immediately undergo rearrangement by application of a small amount of heat, thereby possibly causing recrystallization readily.
The IPC standards also include classification of copper foils from another aspect; i.e., surface profile (roughness) of copper foil which is laminated with a substrate to produce copper-clad laminates. The classification is determined by surface roughness obtained in accordance with the IPC-TM-650 test method. Specifically, copper foils are categorized into three types: standard profile foil (S type) having no particular specified roughness; low profile foil (L type) having a maximum roughness of 10.2 xcexcm or less; and very low profile foil (V type) having a maximum roughness of 5.1 xcexcm or less.
Among them, when a copper foil belonging to V type, setting aside S type or L type, is produced by electrolysis, the amounts of impurities in an electrolytic solution must be lowered and conditions for electrolysis must be particularly tailored. The grain size of deposited crystals must be reduced to a considerably small size such that the grains cannot be observed under an optical microscope having a magnification of some 100 times, as compared with columnar deposits typically observed under an optical microscope. Thus, electrodeposited copper foil belonging to V type has very fine crystal grains, and such metallographic structure is completely different from that of other copper foils. The fine crystal grains provide high tensile strength and hardness.
The aforementioned difference in metallographic characteristic provides difference in physical properties of copper foil, and coefficient of thermal expansion varies to a small deg.C in accordance with the aforementioned types of copper foils. Therefore, when an electrodeposited copper foil endowed with appropriate physical properties; particularly, appropriate coefficient of thermal expansion, is employed as a carrier foil of electrodeposited copper foil with carrier, the coefficient of thermal expansion can be controlled to a value different from that of an electrodeposited copper foil of the electrodeposited copper foil with carrier.
In the present invention, the electrodeposited copper foil belonging to Grades 1 to 3 of the IPC standards and for forming the carrier foil layer refers to the aforementioned standard electrodeposited copper foil, HTE foil, and S-HTE foil. The material for forming the electrodeposited copper foil layer is a copper foil having very fine crystal grains and categorized into very low profile type (V type) of the IPC standards. Coefficient of thermal expansion (xcex1) of these copper foils were measured, and the results are shown in Table 1. In Table 2, absolute values of the difference between coefficient of thermal expansion (xcex1) of an electrodeposited copper foil layer and that (xcex1) of a carrier foil layer are summarized. Coefficient of thermal expansion was measured by means of a thermo-mechanical analyzer, TMA standard type CN8098F1 (product of Rigaku Denki).
Calculated absolute values of (xcex1 of electrodeposited copper foil layer)xe2x88x92(xcex1of carrier foil) are shown in Table 2. When S-HTE foil is employed as a carrier foil, the average absolute value of the difference in coefficient of thermal expansion is 0.046xc3x9710xe2x88x925/deg.C in the temperature-elevating step and 0.049xc3x9710xe2x88x925/deg.C in the temperature-lowering step. When HTE foil is employed as a carrier foil, the average absolute value of the difference in coefficient of thermal expansion is 0.268xc3x9710xe2x88x925/deg.C in the temperature-elevating step and 0.318xc3x9710xe2x88x925/deg.C in the temperature-lowering step. When standard electrodeposited copper foil belonging to Grade 1 is employed as a carrier foil, the average absolute value of the difference in coefficient of thermal expansion is 0.225xc3x9710xe2x88x925/deg.C in the temperature-elevating step and 1.205xc3x9710xe2x88x925/deg.C in the temperature-lowering step.
When the carrier foil layer and the electrodeposited copper foil layer are subjected to heat hysteresis and the two layers exhibit identical thermal expansion behavior, the bonding conditions between the two layers via the organic adhesive interface layer are maintained within an elastic limit. Under such conditions, peeling at the organic adhesive interface layer is not promoted. Briefly, the greater the difference in coefficient of thermal expansion, the more easily peeling occurs due to thermal expansion, and the smaller the difference in coefficient of thermal expansion, the more difficult peeling is. In order to elucidate the relationship between coefficient of thermal expansion and peel strength, the data must be compared in the aforementioned temperature range. The difference in coefficient of thermal expansion must be 4xc3x9710xe2x88x927/deg.C or more. As is clear from Table 2, the difference in coefficient of thermal expansion is smaller in the temperature-elevating step than in the temperature-lowering step. Accordingly, whenever the coefficient of thermal expansion falls within the above range in the temperature-elevating step, it is considered to fall within the above range also in the temperature-lowering step.
From the test results of samples employing three types of carrier foils, carrier foil is considered to be easily peeled when HTE foil or standard electrodeposited copper foil is employed as the carrier foil. The reason for the low peel strength is that the difference between the coefficient of thermal expansion of V-type copper foil serving as the electrodeposited copper foil layer and that of carrier foil increases when HTE foil or standard electrodeposited copper foil is employed as the carrier foil instead of S-HTE foil. Since S-HTE foil is recrystallized at approximately 180xc2x0 C., the S-HTE foil easily follows the thermal expansion behavior of the electrodeposited copper foil layer during heating as compared with HTE foil. Thus, peeling at the organic adhesive interface layer is considered to be suppressed. Briefly, the greater the difference in coefficient of thermal expansion, the more easily peeling occurs due to thermal expansion.
The above-shown data are typical data among the data which the present inventors have obtained in their research. Thus, electrodeposited copper foils with carrier which are formed of the materials satisfying the above conditions can exhibit a peel strength of 3 gf/cm to 100 gf/cm; i.e., the target peel strength of the present invention, after hot-pressing for producing copper-clad laminates is completed. In addition, the present inventors have carried out further experiments, and have found that when the average difference between the coefficient of thermal expansion of the electrodeposited copper foil layer at a certain temperature and that of the carrier foil layer at the same temperature in the temperature-elevating step is 0.04xc3x9710xe2x88x925/deg.C or more, the target peel strength of the carrier foil can be attained.
Thus, when electrodeposited copper foil belonging to any one of Grades 1 to 3 is employed as the carrier foil and V-type foil is employed as the electrodeposited copper, the average difference between the coefficient of thermal expansion of the electrodeposited copper foil layer at a certain temperature and that of the carrier foil layer at the same temperature becomes 0.04xc3x9710xe2x88x925/deg.C or more. As a result, the carrier foil can be peeled at a peel strength of 3 gf/cm to 100 gf/cm after hot-pressing for producing copper-clad laminates is completed.
In the present invention, at least one species selected from nitrogen-containing organic compounds, sulfur-containing organic compounds, and carboxylic acids is preferably employed as the organic agent. The specific organic agents described below are suitably used in the present invention. At present, it is confirmed that these compounds are not detrimental to production of printed wiring boards from produced copper-clad laminates including steps such as resist-application steps, etching steps, plating steps, and mounting steps.
Among these compounds, the nitrogen-containing organic compounds may have a substituent. Specifically, substituted triazloes are preferably used. Examples include 1,2,3-benzotriazole (hereinafter referred to as BTA), carboxybenzotriazole (hereinafter referred to as CBTA), Nxe2x80x2, Nxe2x80x2-bis(benzotriazolylmethyl) urea (hereinafter referred to as BTD-U), 1H-1,2,4-triazole (hereinafter referred to as TA), and 3-amino-1H-1,2,4-triazole (hereinafter referred to as ATA).
Examples of preferably employed sulfur-containing compounds include mercaptobenzothiazole (hereinafter referred to as MBT), thiocyanuric acid (hereinafter referred to as TCA), and 2-benzimidazolethiol (hereinafter referred to as BIT).
Monocarboxylic acids are particularly preferably used as the carboxylic acids. Examples include oleic acid, linoleic acid, and linolenic acid.
Throughout the description, the term xe2x80x9celectrodeposited copper foil (electrodeposited copper foil layer)xe2x80x9d refers to an electrodeposited copper foil coated with copper microparticles for anchoring and an anti-corrosion layer as shown in the cross-sectional view of FIG. 2. The copper microparticles form a surface-treated layer which ensures stable adhesion between an insulating substrate and a bulk copper layer for maintaining electrical conductivity of the produced printed wiring boards. However, in the present description, detailed description of the surface-treated layer is omitted in the parts other than xe2x80x9cModes for Carrying Out the Invention.xe2x80x9d
The aforementioned electrodeposited copper foil with carrier is produced by a method including forming an organic adhesive interface layer on a carrier foil by use of an organic agent and electrodepositing copper serving as an electrodeposited copper foil layer.
In the invention, there is provided a copper-clad laminate which is produced from an electrodeposited copper foil with carrier. The carrier foil of the copper-clad laminate can be peeled readily and smoothly by considerably low peeling force, thereby further enhancing operational efficiency. In addition, the carrier foil can be peeled stably at 3 gf/cm to 100 gf/cm, thereby attaining automated peeling operation by means of a peeling machine.